1. Field of the Invention
The invention is to cooling a coil by shaving off the insulation from an area on the conductor and contacting the bared conductor area with a thermally conductive electrically insulative ceramic to transfer heat away from the conductor and coil.
2. Description of Related Art
Large power motors generate proportionally large amounts of heat. Their conductors, usually copper, are insulated by thin electrically insulating coatings or tape. These coatings, while effective as electrical insulators, unfortunately are also thermally insulative, and obstruct free flow of heat out of the conductors. As a result, the conductors used in the prior art motors see a large rise in temperature. The situation is worsened further when the coil is made by packing a large number of conductors inside a coil window or iron slot. The conductors remote from the cooler window boundaries (those near the center of the coil window) are hotter than those close to the wall of the window. Such hotter conductors are referred to as hot spots.
Hot spot conductors are the first to fail because of thermal degradation of insulation. If the hot spot temperature exceeds the temperature rating of the insulation, it will start failing. The power rating of the motor is limited to ensure that the hot spot temperature never exceeds the thermal rating of the insulation coating. Methods that can reduce hot spots are needed in order to achieve higher power density and longer coil life. These methods are broadly grouped into those that reduce dissipated power and those that reduce thermal resistance.
In the prior art, several methods are used to reduce thermal resistance. These methods include the use of thinner material to reduce thermal gap, more thermally conductive materials that reduce thermal resistence, extended surfaces to increase heat transfer area, using thermally conductive potting compound, eliminating air voids by vacuum pressure impregnation, etc.
E. Sines (U.S. Pat. No. 6,777,835, issued Aug. 17, 2004) proposed inserting an electrically and thermally conductive strip between conductors such as a pitch-graphite composite strip. Its thermal conductivity is so high that it can redirect the heat along its narrow section. Its thermal conductivity is ˜600 w/mK which allows heat to flow in a thinner section without increasing resistance. However, this material is electrically conductive, and had to be covered by two electrically insulating layers. The heat has to overcome the resistance of the insulation layers. Thus, even though pitch composite has high thermal conductivity, its insulation layer will obstruct heat flow, increasing thermal resistance.
E. Jarczynski (U.S. Pat. No. 5,091,666, issued Feb. 25, 1992) proposed inserting bare copper strips between laminations to reduce the temperature rise in iron laminations. Such copper strips however are in the path of alternating fields and hence generate eddy losses.
Liebe et al (U.S. Pat. No. 3,965,378) proposed inserting bare copper strips that protrude out of the coil; cool air grazes over it to remove heat by free convection. However, it is well known that the boundary layer of free convective air is about 0.2 in. thick. Packing of copper protrusions at less than 0.25 in. spacing, as proposed by Liebe, will degrade cooling effectiveness.
Liang et al (U.S. Pat. No. 6,744,158, issued Jun. 1, 2004) teaches using conducting rings around the coils for cooling. T. Nilson (U.S. Pat. No. 6,798,105, issued Sep. 28, 2004) teaches insulated winding wires in contact with a cooling medium. R. Nygard (U.S. Pat. No. 5,886,434, issued Mar. 23, 1999) and P. Eckels (U.S. Pat. No. 4,282,450, issued Aug. 4, 1981) are examples of fluid cooling of electrical windings.